COOTAMUNDRA-GUNDAGAI REGIONAL COUNCIL COOTAMUNDRA FLOOD STUDY DRAFT FOR PUBLIC EXHIBITION NOVEMBER 2020 Level 2, 160 Clarence Street Sydney, NSW, 2000 Tel: (02) 9299 2855 Fax: (02) 9262 6208 Email: [email protected] Web: www.wmawater.com.au COOTAMUNDRA FLOOD STUDY DRAFT FOR PUBLIC EXHIBITION NOVEMBER 2020 Project Project Number Cootamundra Flood Study 119039 Client Client’s Representative Cootamundra-Gundagai Regional Council Mike Brearley Project Manager Erin Askew Revision History Revision Description Distribution Authors Reviewed Verified Date by by 0 Stage 1 – Data Collection CGRC DPIE CW, DB EA EA AUG 19 1 Stage 2- Model Build and CGRC DPIE DB EA EA JUN 20 Calibration 2 Stage 3 – Draft Flood Study CGRC DPIE DB, EA EA EA AUG 20 Report 3 Draft for Public Exhibition CGRC DPIE DB, EA EA EA NOV 20 Cover photo: Bradman Oval and Cutler Avenue Causeway, September 2016, (Photo B Cant) COOTAMUNDRA FLOOD STUDY TABLE OF CONTENTS PAGE LIST OF ACRONYMS ............................................................................................................. viii ADOPTED TERMINOLOGY ...................................................................................................... ix FOREWORD ............................................................................................................................. xi 1. INTRODUCTION ........................................................................................................ 1 2. BACKGROUND ......................................................................................................... 2 Study Area .................................................................................................. 2 Nature of Flooding at Cootamundra ............................................................ 3 Historic Flood Events .................................................................................. 3 2.3.1. January 1885 .............................................................................................. 4 2.3.2. April 1903 ................................................................................................... 4 2.3.3. December 1919 .......................................................................................... 4 2.3.4. Floods of the 1950s .................................................................................... 5 2.3.5. January 1974 .............................................................................................. 5 2.3.6. August 1983 and January 1984 .................................................................. 5 2.3.7. December 2010 .......................................................................................... 6 2.3.8. September 2016 ......................................................................................... 6 3. PREVIOUS STUDIES ................................................................................................. 8 Cootamundra Flood Study Report, NSW Water Resources Commission, 1986 (Reference 5) ..................................................................................... 8 Cootamundra Lake Flood Study, Maunsell Pty Ltd, 1997 (Reference 6) ..... 9 Cootamundra Floodplain Management Study and Plan, Willing & Partners, 2001 (Reference 7) ................................................................................... 11 Jindalee Creek Levee, Cootamundra, Preliminary Design Report, Cardno Willing, August 2004, (Reference 8) ......................................................... 13 Cootamundra Local Flood Plan, NSW SES, June 2007, (Reference 9) .... 14 Stormwater Priority Assessment Report, Brearley & Hansen, 2018 (Reference 10) .......................................................................................... 14 Survey and Design of Six Stormwater Improvement Projects, Design Report, 2019, (Reference 11) ................................................................... 15 4. AVAILABLE DATA .................................................................................................. 16 Aerial Imagery .......................................................................................... 16 Topographic Data ..................................................................................... 16 4.2.1. LiDAR ....................................................................................................... 16 4.2.2. Muttama Creek Cross Section Survey ...................................................... 16 4.2.3. Boundary Road Subdivision ...................................................................... 17 GIS Layers ............................................................................................... 17 Hydraulic Structures ................................................................................. 17 4.4.1. In-Field Measurements ............................................................................. 17 4.4.2. Previous Survey ....................................................................................... 17 4.4.3. Notes about upgraded structures .............................................................. 18 4.4.4. Pit and Pipe Network ................................................................................ 18 4.4.5. Details of railway structures from ARTC ................................................... 19 Site Visit ................................................................................................... 19 Land Use in the Study Area ...................................................................... 20 Floor Level Database ................................................................................ 20 Flood Marks for Calibration ....................................................................... 21 Gauge Data in the Study Area .................................................................. 22 119039: R201124_Cootamundra_FS_PublicExDraft.docx: 24 November 2020 ii 4.9.1. Stream Gauges ........................................................................................ 22 4.9.2. Rainfall Gauges ........................................................................................ 22 4.9.3. Analysis of September 2016 Rainfall Event .............................................. 23 4.9.4. Analysis of March 2012 Rainfall event ...................................................... 24 4.9.5. Analysis of December 2010 Rainfall event ................................................ 25 4.9.6. Analysis of March 2010 Rainfall event ...................................................... 26 4.9.7. Analysis of January 1984 Rainfall Event ................................................... 27 4.9.8. Analysis of January 1974 Rainfall Event ................................................... 28 Historic Rainfall Data ................................................................................ 28 4.10.1. Limitations of using historic rainfall data ................................................. 29 Design Flood Inputs .................................................................................. 30 4.11.1. Design Rainfall Data .............................................................................. 30 4.11.2. Design Rainfall Losses .......................................................................... 31 4.11.3. Design Temporal Patterns ..................................................................... 32 4.11.4. Areal Reduction Factor Parameters ....................................................... 33 5. STAKEHOLDER ENGAGEMENT ............................................................................ 34 Floodplain Risk Management Committee ................................................. 34 Community Consultation ........................................................................... 34 5.2.1. Drop-in Session ........................................................................................ 35 5.2.2. Questionnaire Results .............................................................................. 35 Public Exhibition ....................................................................................... 36 6. HYDROLOGIC MODEL SETUP ............................................................................... 37 Introduction ............................................................................................... 37 Sub-catchment delineation ....................................................................... 37 Impervious Surface Area .......................................................................... 38 Rainfall Loss ............................................................................................. 39 WBNM Parameters ................................................................................... 39 7. HYDRAULIC MODEL SETUP .................................................................................. 40 Introduction ............................................................................................... 40 DEM and Grid Resolution ......................................................................... 40 TUFLOW Hydraulic Model Extent ............................................................. 41 Boundary Locations .................................................................................. 41 7.4.1. Inflows ...................................................................................................... 41 7.4.2. Downstream Boundary ............................................................................. 41 Surface Roughness .................................................................................